This invention relates to stepper motor driving systems and more particularly to means for damping a stepper motor by controlling its driving current.
Most stepper motors have a salient pole stator structure carrying a number of windings, one to each pole with oppositely located windings being generally connected together into one single motor phase winding, whilst rotors fall into two categories: the so called permanent magnet and variable reluctance categories. Driving is performed in both categories by controlling the application of current to predetermined motor phase windings with predetermined sets of binary valued signals. In other words, the motor is driven by energizing the motor phase windings in an order and at specific times determined by the motor characteristics and the specific motor move desired.
Stepper motors are normally used for driving mechanical loads requiring fast and precise positionings repeated at a relatively high rate. However, when a stepper motor stops, it oscillates about its rest position until these oscillations decrease, due to inherent damping, and finally stop. If it is desired to stop in some position within some tolerance band before taking any further action regarding the system load, and the oscillations are larger than the tolerance band, then the system is delayed until the oscillation amplitude reaches an acceptable level. A damping ratio of the order of 0.01 is typical for conventional stepper motors presently available. Such an inherent damping is by itself inadequate for high speed operation of the stepper motor driven system.
Another drawback of the stepper motor driven system is also connected to the oscillations mentioned above and occurs when a series of motor moves are made with a relatively short time between moves. In this case, the oscillations continue through the whole series of moves and can build up to the point that the motor loses synchronism (break phase). This phenomenon is particularly severe since the motor will not end up in the desired position. This is obviously most detrimental for open-loop type of configurations where no feedback is used for correcting the motor positioning based on actual position. With open-loop configurations, any phase breaking is carried on subsequently and results in false subsequent load positionings without any special means for detecting the erroneous positionings being provided.
Means have been provided for increasing the motor damping under control. The most common method for increasing said motor damping calls for the use of additional stator resistances. The use of resistance leads to energy dissipation which should be avoided, if possible, for obvious reasons such as heating effects and inefficient operation.
Other purely electronic solutions have been disclosed, which are based on the so called "retrotorque" principle in which damping is performed by applying brief pulses to phases just previously energized. Even though these methods eliminate the need for a damping resistor, they require fairly elaborate electronic equipment and most of all do not provide means for conveniently controlling the inherent damping. That is, the damping which would be observed for simple oscillation about rest position.
Other methods have also been provided for eliminating the damping resistor.
U.S. Pat. No. 4,140,955 to R. B. Drabing is one of these. This patent calls for an electronic system wherein a driving circuit is provided which produces an excitation cycle wherein each field winding is first excited with a relatively high, constant DC voltage causing an increase in current. Thereafter, the high voltage across each field winding is pulse modulated with a duty cycle proportional to the ratio of the desired operating voltage to the voltage of the power source. This system operates like a dual power-supply stepping motor without any need for a second distinct power supply element. No improvement on inherent damping is achieved.
U.S. Pat. No. 4,035,708 to U. Schaff calls for means for reducing the power loss of a stepping motor when said motor is at standstill by lowering the winding current from its nominal value to a substantially lower holding value when the motor is stopped. The current is turned on and off during normal operation. The phase current sensed value is used to trigger a trigger circuit which in turn controls ON-OFF turning of the power supply driving the motor. This patent addresses only power saving problems.
Application Ser. No. 57,777, filed July 16, 1979, entitled "Electronic Damping Of Stepper Motor" and assigned to the assignee of this invention, calls for an electronic damping process based on current chopping with a reference voltage varying in accordance with the motor speed. This system requires the use of a transducer and in addition does not call for relatively inexpensive and yet reliable means for making the damping ratio easily adjustable to the actual motor operating mode.